THE DESIGN PROTOCOL: PART 1, SPEAKER CONFIGURATION

Speaker configuration addresses serious aspects of the system under design : driver selection, cabinet size, special crossover properties, overall cost. One must carefully consider input information so that all these aspects be defined effectively.

-Isn't it enough to say that we wish all operation parameters of our speaker under design to reach maximum quality ?

No we can't say that, because it can not be done. We have already stated that speakers are closely related to their environment. A reference high-cost three-way 80-liter giant speaker can not operate in a teenager room ! Speakers are "operation-specific" devices.

PART 1:

- INPUT CONSTRAINTS:

- MAJOR DELIVERABLES:

- special use ?

-> speaker volume

- room size ?

-> cabinet requirements

- nominal impedance ?

-> number of acoustic branches (ways)

- power handling capacity ?

-> number of woofers

- cost level ?

-> woofer cost level

- MINOR DELIVERABLES:

-> special crossover tasks

STEP 1:

SELECT SPEAKER CATEGORY - TARGET LISTENING SPACE

-> SPEAKER VOLUME

-> ADDITIONAL PROPERTIES

special use

near-field monitor

small volume

high quality drivers

speaker resting on a shelf

or flush mounted on a wall

very small volume

height no more than 30cm, special

crossover tasks

desktop speaker connected to a computer

very small volume

low power drivers

corner placement

whatever

better be closed box design

hometheater, center speaker

small volume

2-way, 2 woofers, horizontally directive,

D'Appolito configuration, closed box.

hometheater, rear satellite speaker

small or medium volume

typical domestic use

small-sized room, <7m2

very small volume

medium-sized room,8-16m2

small volume

average-sized room,17-29m2

medium volume

large living room,>30m2

large volume

STEP 2:SELECT SPEAKER VOLUME

SELECT NOMINAL IMPEDANCE

-> WOOFER CONFIGURATION

->2 OR 3-WAY)

-> ADDITIONAL PROPERTIES

very small speaker volume, <12 lt

system at 8 Ohms

single 5.5 inch woofer

2-way

system at 4 Ohms

two 4-4.5 inch woofers

2-way

increased vertical directivity

small speaker volume, 12-20 lt

system at 8 Ohms

single 6.5 inch woofer

2-way

system at 4 Ohms

two 4.5-5 inch woofers

2-way

increased vertical directivity

average speaker volume, 20-35 lt

system at 8 Ohms

single 8 inch woofer

2 or 3-way

system at 4 Ohms

two (2) 6.5 inch woofers

2 or 3 way

increased vertical directivity

large speaker volume, >35 lt

system at 4 or 8 Ohms

single 10 or 12 inch woofer

3 way

system at 4 Ohms

two 8 or 10 inch woofers

3 way

increased vertical directivity

STEP 3:SELECT TOTAL POWER HANDLING CAPACITY

-> WOOFER COST LEVEL / SETUP / CABINET REQUIREMENTS

->SPECIAL CROSSOVER TASKS

<30 Watt RMS per channel

low cost drivers

no need for impedance compensation

<6.5'' single woofer or two 4.5-5'' woofers

enclosure wall thickness can be less than 15mm

30-50 Watt RMS per channel

medium cost tweeter

no need for impedance compensation

6.5-8'' single medium-cost woofer or two 6.5'' low-cost woofers

enclosure wall thickness should be 15mm or more

50-100 Watt RMS per channel

quality drivers

better be be impedance compensated

6.5-8'' single woofer or two 6.5'' woofers

enclosure wall thickness should be 19mm or more, internal bracing could be considered

>100 Watt RMS per channel

quality drivers

better be impedance compensated

10-12'' single woofer or two 8'' woofers

enclosure wall thickness should be 22mm, internal bracing mandatory

The majority of woofer, squawker and tweeter drivers have an SPL sensitivity of 90 dB (per 2.83 Vrms of input signal) approximately. This means that there are some tweeters with a 91-92dB SPL sensitivity value but most of tweeters have the common 90dB value.

So in a 3-way project with one 10 or 12'' woofer we must choose a midrange and a tweeter driver with sensitivities similar to that of our woofer. If we have two large diameter woofers we will not be able to find a midrange and a tweeter driver with the resulting sensitivity value : 96-98 dB. In such a case we should look for woofers of very low sensitivity (84-86dB).

In single 8'' woofer projects the most common sensitivity is that of 90dB. This value holds from 400-500Hz up to 3-4kHz. Below 350 Hz diffraction step shifts SPL levels down. Depending on our preferences over crossover design targets we may choose our low-pass filter to give a smooth rolloff above 400-500 Hz. In this way the effective woofer sensitivity will be further reduced by 2-4 dB and all we have to do is to avoid choosing a 91-92 dB midrange or tweeter.

In two 8'' woofer projects (where the resulting unfilteres sensitivity level at 400-500Hz will be 95-96 dB) the low-pass filter rolloff trick will create a filtered woofer sensitivity of 91-92 dB which is quite manageable: a midrange and a tweeter driver of similar values can be easily selected in the market.

In single 6.5'' or less woofer projects the nominal woofer sensitivity value is furnished at a rather high (and narrow) frequency range. It is a rule of thumb to choose tweeters of equal or even slightly less sensitivity value. SPL matching is then assigned to the crossover filter responses.

In 2 X 6.5'' or less woofer projects we can safely use a tweeter of 90dB sensitivity and let again our crossover filters do the necessary SPL trimming of the 300-1000 Hz frequency range.